The present invention relates to a positive electrode active material containing lithium and nickel as primary components and a non-aqueous electrolyte secondary cell using the above positive electrode active material.
In recent year, various portable electronic devices, such as camcorders, mobile phones, and laptop computers, have been introduced to the market and have been increasingly in demand. Concomitant with the trend toward compact and light-weight electronic devices, research and development of cells, in particular, secondary cells, used as a portable electrical power source has been actively carried out in order to increase an energy density. Compared to related aqueous electrolytic solution secondary cells, such as a lead cell, a nickel-cadmium cell, and a nickel-hydrogen cell, since a lithium ion secondary cell has a high energy density, the demand thereof is large, and in addition, when environment resistance of this secondary cell is improved, expansion of the application thereof can be further expected. Furthermore, concomitant with the trend toward electronic devices having higher functionality, the power consumption tends to increase, and hence excellent performance of discharging a large current has also been required.
As positive electrode active materials used for a lithium ion cell, for example, a lithium cobalt composite oxide having a layered rock-salt structure, a lithium nickel composite oxide, and a lithium manganese composite oxide having a spinel structure have been practically used. Although individual oxides have their own particular features, since having well-balanced properties in view of capacity, cost, thermal stability, and the like, a lithium cobalt composite oxide has been widely used in recent years. A lithium manganese compound oxide has a low capacity and slightly inferior high-temperature storage properties. In addition, since having slightly inferior crystal structure stability and causing decomposition of an electrolyte by a side reaction, a lithium nickel compound oxide disadvantageously has inferior cycle properties and environmental resistance. However, in view of prices of starting materials and supply stability, the composite oxides described above are superior to a lithium cobalt compound oxide, and hence intensive research has been implemented focusing on future application and expansion.
As for a lithium nickel compound oxide, the following methods have been proposed in order to overcome the above problems. There may be mentioned a method (1) in which the cycle properties are improved by replacing part of nickel with another element (for example, see Japanese Unexamined Patent Application Publication Nos. 5-283076, 8-37007, and 2001-35492); a method (2) in which a particular metal salt or the like is added (for example, see Japanese Unexamined Patent Application Publication No. 7-192721); and a method (3) in which a binder in a positive electrode active material is defined (for example, see Japanese Unexamined Patent Application Publication No. 10-302768). However, according to research carried out by the inventor of the present invention, environmental resistance, in particular, properties under high temperature environment obtained by the methods (1) to (3) were not satisfactory.
In addition a method (4) has been proposed in which surfaces of grains of a positive electrode active material are covered with a conductive agent or another layered oxide (for example, see Japanese Unexamined Patent Application Publication Nos. 7-235292, 11-67209, and 2000-149950). However, according to the research carried out by the inventor of the present invention, it was confirmed that by the method (4) described above, the capacity is decreased, and discharge properties of discharging a large current are degraded. Hence, it is difficult to apply the method (4) described above to a cell which is required to have a large capacity and a large electricity.
Furthermore, a method (5) has been disclosed in which a metal or a metal oxide, which is unlikely to decompose an non-aqueous electrolyte, is dispersed and held on surfaces of grains of a positive electrode active material (for example, see Japanese Unexamined Patent Application Publication No. 8-102332). However, according to the research carried out by the inventor of the present invention, it was also confirmed that since the metal and the metal oxide dispersed on the surfaces have no lithium ion conductivity according to the method (5) described above, intercalation of lithium ions into a positive electrode active material and deintercalation therefrom are inhibited, and that discharge properties of discharging a large current are particularly degraded. In addition, the amount of the dispersed material disclosed in the method described above was not large enough to obtain the effect described above.
In addition, a method (6) has also been disclosed in which surface layers containing titanium are formed on grains of a positive electrode active material (for example, see Japanese Unexamined Patent Application Publication No. 2002-63901). However, according to the research carried out by the inventor of the present invention, by the method (6) described above, it was found that a sufficient effect of improving properties at high temperature operation cannot be obtained.
As described above, it has been difficult to simultaneously improve the properties (hereinafter referred to as “high temperature-operation properties) at high temperature operation and the discharge properties (hereinafter referred to as “large current-discharge properties) of discharging a large current. Hence, a non-aqueous electrolyte secondary cell has been desired which has both superior high temperature-operation properties and excellent large current-discharge properties.